The present invention relates generally to radio communication systems and, more specifically, to methods for interference suppression to improve the quality of signals received in mobile stations.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry's growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
Throughout the world, one important step in the advancement of radio communication systems is the change from analog to digital transmission. Equally significant is the choice of an effective digital transmission scheme for implementing the next generation technology. Furthermore, it is widely believed that the first generation of Personal Communication Networks (PCNs), employing low cost, pocket-sized, cordless telephones that can be carried comfortably and used to make or receive calls in the home, office, street, car, etc., will be provided by, for example, cellular carriers using the next generation digital cellular system infrastructure. An important feature desired in these new systems is increased traffic capacity.
Currently, channel access is primarily achieved using Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) methods. In FDMA, a communication channel is a single radio frequency band into which a signal's transmission power is concentrated. Signals which can interfere with a communication channel include those transmitted on adjacent channels (adjacent channel interference) and those transmitted on the same channel in other cells (co-channel interference). Interference with adjacent channels is limited by the use of band pass filters which only pass signal energy within the specified frequency band. Co-channel interference is reduced to tolerable levels by restricting channel re-use by providing a minimum separation distance between cells in which the same frequency channel is used. Thus, with each channel being assigned a different frequency, system capacity is limited by the available frequencies as well as by limitations imposed by channel reuse. FDMA was used for channel access in first generation systems such as AMPS.
In TDMA systems, a channel consists of, for example, a time slot in a periodic train of time intervals over the same frequency. Each period of time slots is called a frame. A given signal's energy is confined to one of these time slots. Adjacent channel interference is limited by the use of a time gate or other synchronization element that only passes signal energy received at the proper time. Thus, with each channel being assigned a different time slot, system capacity is limited by the available time slots as well as by limitations imposed by channel reuse as described above with respect to FDMA. TDMA has been used to provide channel access for second generation radiocommunication systems, such as D-AMPS.
With FDMA and TDMA systems (as well as hybrid FDMA/TDMA systems), one goal of system designers is to ensure that two potentially interfering signals do not occupy the same frequency at the same time. In contrast, Code Division Multiple Access (CDMA) is a channel access technique which allows signals to overlap in both time and frequency. CDMA is a type of spread spectrum communications, which have been around since the days of World War II. Early applications were predominantly military oriented. However, today there has been an increasing interest in using spread spectrum systems in commercial applications since spread spectrum communications provide robustness against interference, which allows for multiple signals to occupy the same bandwidth at the same time. Examples of such commercial applications include digital cellular radio, land mobile radio, and indoor and outdoor personal communication networks.
In a typical CDMA system, an information data stream to be transmitted is impressed upon a much-higher-bit-rate data stream produced by a pseudorandom code generator. The information signal and the pseudorandom signal are typically combined by multiplication in a process sometimes called coding or spreading the information signal. Each information signal is allocated a unique spreading code. A plurality of coded information signals are transmitted as modulations of radio frequency carrier waves and are jointly received as a composite signal at a receiver. Each of the coded signals overlaps all of the other coded signals, as well as noise-related signals, in both frequency and time. By correlating the composite signal with one of the unique spreading codes, the corresponding information signal can be isolated and decoded.
Transmit power control methods are important to CDMA communication systems having many simultaneous transmitters because such methods reduce the mutual interference of such transmitters. Depending upon the system characteristics, power control in such systems can be important for the uplink (i.e., for transmissions from a remote terminal to the network), the downlink, (i.e., for transmissions from the network to the remote terminal) or both. Like TDMA, CDMA has been used to provide channel access in some later developed second generation systems, such as IS-95.
As information technologies and communication technologies continue to grow closer together, demand for high data rate support (e.g., greater than 56 kbit/s) is rapidly increasing, particularly with the advent of the Internet and the desire to transmit video information. Second generation radiocommunication systems were not designed to handle such high data rates. Accordingly, third generation systems are now under development, for which both TDMA and wideband CDMA are being considered for channel access.
One of the features of a wideband CDMA cellular system, compared to today's narrowband systems, is the potential to support data communication with high data rates, e.g., 384 kbit/sec. Mobile stations communicating at such high data rates will, however, consume a great deal of the system capacity. To reduce the impact of high data rate users on system capacity, these users may be required to operate on signals which have a lower signal-to-noise ratio (E.sub.b .vertline.N.sub.0), i.e, their receivers may be required to handle more interference.
Moreover, the introduction of so-called "home" base stations into radiocommunication systems may create situations where new types of localized, low signal-to-noise ratio, signals exist. The home base station concept involves providing users with base station units in their homes which their mobile units can communicate with to place calls when the users are at home. Access to these home base stations is intended to be restricted to only authorized home users. Thus, other units connected to the radiocommunication which pass by the home base station(s) may experience additional localized interference and a corresponding reduction in signal-to-noise ratio for the signal supporting their connection.
Meeting system requirements to handle less robust signals could be achieved by introducing a more advanced receiver algorithm (i.e., to provide improved detection of relatively fainter signals). However, there are limits on the ability of receiver algorithms to resolve symbols in signals having very low E.sub.b .vertline.N.sub.0. Applicants have recognized that another possible solution lies in using receiver diversity, i.e., multiple antennas at the mobile station, and combining the received signals to overcome the reduction in E.sub.b .vertline.N.sub.0.
Antenna diversity techniques are based on the knowledge that when the path lengths that signals traverse over the transmission medium are relatively small, the multiple signal images arrive at almost the same time. The images add either constructively or destructively, giving rise to fading, which typically has a Rayleigh distribution. When the path lengths are relatively large, the transmission medium is considered time dispersive, and the added images can be viewed as echoes of the transmitted signal, giving rise to intersymbol interference (ISI).
Fading can be mitigated by having multiple receive antennas and employing some form of diversity combining, such as selective combining, equal gain combining, or maximal ratio combining. Diversity takes advantage of the fact that the fading on the different antennas is not the same, so that when one antenna has a faded signal, chances are the other antenna does not.
The usage of multiple antennas in radiocommunication systems for diversity is, per se, known. For example, an algorithm used to process signals received via an array antenna in a base station is described in Naguib, Ayman F. et al., Recursive Adaptive Beamforming for Wireless CDMA, IEEE 1995. However, like that described in Naguib, most applications of antenna diversity have been practiced at the base station for receiving signals transmitted by mobile stations on the uplink. Very few systems have implemented mobile stations having multiple antennas.
One example of antenna diversity at the mobile station, i.e., for processing signals transmitted by the base station on the downlink, is found in the Japanese PDC system. The PDC system uses so-called switch diversity, whereby the receiver in the mobile station selects either of the two versions of the signal coupled to its antennas. This form of diversity is, however, rather simplistic since it does not combine the received signals and, therefore provides relatively low performance. Moreover, this prior implementation of diversity at the mobile station failed to address the significant localized interference problems which will be presented by the home base stations and the upcoming need for higher data transmission rates.
Accordingly, there exists a need to provide techniques for handling the combining of signals received at mobile stations in radiocommunication systems and, more particularly, for handling the processing of plural signals received via antenna array elements at a mobile station.